Now THAT is bizarre!
Was it left or right side of the chain that lost the plates?
Does your chainset have a chain guard/ bash ring type thingmebob as well as the chainring fitted?
It was the side facing into the bike, so I think that would be left! I don't have any chain guards fitted, maybe I should.
Those are INNER plates!!! when I chain brakes it is (nearly) always an outer plate that 'springs' off a pin shortly followed by it opposite number. Have you checked you chain ring or spur gear for damage. Best guess is that one or other has a damaged tooth that is scoring the inner plates - but this is clutching at straws.
I know! I have only ever had outside plates ping off before, usually while changing a gear on a derailleur bike. I didn't notice anything obviously wrong with the chain wheel/sprocket when I put the spare on last night in the dark and it rode ok this morning. I'll definitely check out the scoring hypothesis.
[had a look this lunchtime and there is no sign of any damaged teeth on front ot back]Alternatively, I must have bottomed out the chain wheel on a rock, but why would this break just the inner links?